Accurate impedance measurements of a “component” used in a high-frequency band are required for characteristics evaluation of that component. Here, a “component” is a two-terminal device, for example, a resistor, a capacitor, an inductor, a diode, or a pattern on a printed circuit board, etc., and shall be referred to as a device under test (DUT).
Conventional methods for measuring the impedance of a DUT using automatic balanced bridge methods are limited to frequencies of no more than approximately 110 MHz. Radio frequency current-voltage (RF I-V) methods are limited to a frequency range of approximately 1 MHz to 3 GHz. With each of these methods, a wide range of impedance can be measured at high accuracy. However, the commercially available apparatuses are limited to frequencies below 3 GHz.
Methods that enable impedance measurement at frequencies above approximately 3 GHz using network analysis methods (S-parameter methods) are known; both a reflection coefficient method and a transmission method are known. In the reflection coefficient method, a vector voltage ratio S11 of an input signal and a reflected signal, that is, a reflection coefficient G with respect to a DUT is measured. In the transmission method, the DUT is connected in series or parallel between two test ports and a vector voltage ratio S21 (b2/a1) of an input signal and an output signal is measured.
Conventionally, in the reflection coefficient method, S11 is measured to determine the impedance Zx of the DUT. In the transmission method, the vector voltage ratio S21 is measured to determine the impedance Zx.
In regard to the reflection coefficient method, FIG. 5(a) shows the principles, wherein a relationship formula between the vector voltage ratio S11 and the impedance Zx of a DUT, and a calculation formula for determining the impedance Zx are given. In the figures, Z0 denotes a characteristic impedance (e.g. 50Ω).
In regard to the transmission method FIGS. 5(b) and 5(c) show the relationship between the vector voltage ratio S21 and the impedance Zx of the DUT. In particular, the formula for determining the impedance Zx for cases of connecting the DUT in series and parallel are given, respectively.
FIG. 6 is a graph showing the relationships between the impedance Zx and the vector voltage ratios S11 and S21 in the respective methods. A solid line indicates the vector voltage ratio S11 (b1/a1) of the input signal a1 and the reflected signal b1 in the reflection coefficient method. Alternate long and short dash lines indicate the vector voltage ratios S21 (b2/a1) of the input signal a1 and the output signal b2 in the respective transmission methods in which the DUT is connected in series and in parallel, respectively. Broken lines indicate the vector voltage ratios S11 (b1/a1) of the input signal a1 and the reflected signal b1 in the transmission methods in which the DUT is connected in series and in parallel, respectively. An abscissa axis indicates the impedance Zx normalized by the characteristic impedance Z0.
Related prior art document is for example, Agilent_Technologies: “Impedance Measurement Handbook,” Nov. 2011 edition (cp.literature.agilent.com/litweb/pdf/5950-3000JA.pdf)